1. Technical Field
The present disclosure relates generally to radio frequency (RF) circuits, and more particularly, active harmonic filters for integrated RF power amplifiers.
2. Related Art
Wireless communications systems find application in numerous contexts involving data transfer over long and short distances alike, and there exists a wide range of modalities suited to meet the particular needs of each. Fundamentally, wireless communications involve a radio frequency (RF) carrier signal that is variously modulated to represent data, and the modulation, transmission, receipt, and demodulation of the signal conform to a set of standards for coordination of the same. Many different mobile communication technologies or air interfaces exist, and various generations of these technologies are deployed in phases.
A fundamental component of any wireless communications system is the transceiver, that is, the combined transmitter and receiver circuitry. The transceiver encodes the data to a baseband signal and modules it with an RF carrier signal. Upon receipt, the transceiver down-converts the RF signal, demodulates the baseband signal, and decodes the data represented by the baseband signal. An antenna connected to the transmitter converts the electrical signals to electromagnetic waves, and an antenna connected to the receiver converts the electromagnetic waves back to electrical signals. A communications link is established when both the transmission and reception of a signal are in accordance with mutual parameters.
Transceivers typically do not generate sufficient power or have sufficient sensitivity for reliable communications standing alone. Thus, additional conditioning of the RF signal at both the transmission end and the reception end is necessary. The circuitry between the transceiver and the antenna that provide this functionality is referred to as the front end, which includes a power amplifier for increased transmission power, and/or a low noise amplifier for increased reception sensitivity. The front end also includes a switch circuit that selectively interconnects the power amplifier and the low noise amplifier to the antenna.
In an actual implementation, the RF power amplifier exhibits some degree of non-linearity in the amplification of the radio frequency carrier signal. This results in harmonics, which are integer multiples of the fundamental operating frequency, being generated. These emissions outside the designated operating frequency are undesirable, as they have the potential to interfere with other wireless systems. Accordingly, there are various regulatory requirements that govern the acceptable spurious emission levels in end products to ensure that different communications devices can co-exist and function properly within the same vicinity. These restrictions are imposed on each of the constituent components, including the transceiver and the front end
Conventional RF power amplifiers thus incorporate harmonic filters to minimize the level of undesirable emissions. One possible way to reduce harmonic emissions is by way of a low-pass filter that rejects all frequencies above the fundamental frequency, while another possibility is a notch filter that rejects only certain harmonics as defined by the particular communications standard. In order for the filter to be able to reject high levels of harmonic frequencies, components having low loss such as capacitors and inductors are utilized. On-die harmonics suppression filters are insufficient, as the achievable performance is limited to the components that can be fabricated thereon. Capacitors with high capacitance values and inductors with high inductance values are challenging to fabricate on-die due to their larger footprints, and attendant increases in cost.
Thus, external filter circuits may be added to prevent strong harmonic signals to reach the antenna, either at the module level, e.g., as a separate circuit element that is packaged together with the RF switch, power amplifier, and low noise amplifier, or at the printed circuit board level, e.g., as a separate component and package mounted to the printed circuit board and interconnected with conductive traces between the pin outs of the separate packages. External filters, however, are expensive, require additional space, and introduce additional transmission path losses.
Accordingly, there is a need in the art for harmonic rejection filters with sufficient rejection levels to meet and exceed stringent spurious emissions limits. There is also a need in the art for harmonic rejection filters that may be implemented on-die without substantial increases in die footprint. It would also be desirable for such filters to consume minimal power, with less loss, and higher harmonic suppression than passive traps.